diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
index 463f798..84168c0 100644
--- a/fs/autofs4/dev-ioctl.c
+++ b/fs/autofs4/dev-ioctl.c
@@ -54,11 +54,10 @@
  * Check a string doesn't overrun the chunk of
  * memory we copied from user land.
  */
-static int invalid_str(char *str, void *end)
+static int invalid_str(char *str, size_t size)
 {
-	while ((void *) str <= end)
-		if (!*str++)
-			return 0;
+	if (memchr(str, 0, size))
+		return 0;
 	return -EINVAL;
 }
 
@@ -138,8 +137,7 @@
 	}
 
 	if (param->size > sizeof(*param)) {
-		err = invalid_str(param->path,
-				 (void *) ((size_t) param + param->size));
+		err = invalid_str(param->path, param->size - sizeof(*param));
 		if (err) {
 			AUTOFS_WARN(
 			  "path string terminator missing for cmd(0x%08x)",